Various types of semi-conductor devices may be fabricated utilizing a crystalline silicon substrate. One such type of device is a silicon solar cell also known as a photovoltaic cell which generates electrical power when exposed to solar radiation from the sun. Other classes of semi-conductor devices which may be fabricated using a crystalline silicon substrate include, but are not limited to, integrated circuit devices such as transistors, photonic devices, piezoelectronic devices, flat panel displays, micro-electromechanical systems, nanotechnology structures and similar components.
Crystalline silicon is generally available in the form of wafers provided in various sizes and thicknesses. The wafers are initially prepared from larger silicon boules. A typical wafer may have a thickness of between about 150 μm to 925 μm. For large-area semiconductor devices such as photovoltaics, the cost of the crystalline silicon substrate is very high. Many such devices may be effectively implemented with a layer of crystalline silicon which is substantially thinner than typical wafer thicknesses. Accordingly, several methods of forming or growing a thinner film of crystalline silicon have been developed.
For example, techniques have been developed for the epitaxial growth of highly ordered crystalline silicon on non-silicon substrates. Epitaxial growth on a non-crystalline substrate is technically challenging since a specialized template or other method must be implemented to guide formation of high-quality silicon on an amorphous or polycrystalline substrate. In addition, many substrates which are good insulators, inexpensive and otherwise desirable, cannot withstand the high temperatures associated with many methods of epitaxial crystalline silicon deposition. It is important to note that a thin film of crystalline silicon must be associated with a substantially more robust and ideally significantly less expensive substrate as a part of the device fabrication process. Naturally, this association occurs automatically if a thin silicon film is grown on the selected substrate, however, for certain devices it may be advantageous to transfer a thin film of crystalline silicon to the secondary substrate after the film has been formed.
Various methods for preparing and transferring a thin film of crystalline silicon have been developed. In one method, particles such as hydrogen, helium or other compounds may be energetically implanted at a selected depth in crystalline silicon substrate material, a wafer for example. For example, hydrogen ions may be selectively implanted into a silicon wafer to a depth of up to about 0.5 μm and with accuracy of about +/−0.05 μm using available ion implantation methods. The implanted particles serve to add stress or otherwise reduce the fracture energy along a plane parallel to the top surface of the substrate at the selected implantation depth. This fabricated stress plane allows for a controlled cleave along the implanted ion plane to remove a thin layer of silicon. The cleaving process itself may be initiated using mechanical, thermal, or optical energy. Before or after the thin film of crystalline silicon is cleaved from the substrate the film may be bound to a secondary substrate, processed or otherwise used for device fabrication.
The ion implementation method of setting a cleavage or fracture zone at a selected depth within the original substrate necessarily causes a portion of the substrate which is equal to or greater in thickness to the removed film to be irretrievably utilized. Accordingly, any selected substrate has a finite number of thin films of crystalline silicon which may be removed before all available material has been utilized. In addition, this method relies upon ion implantation which can be expensive. Furthermore, ion implantation does not leave a perfectly uniform layer of hydrogen or other implanted ion at the cleavage depth. Thus, the surface of the removed thin film may be rough and require subsequent polishing, annealing or cleaning steps. The ion implantation method of fabricating a thin film of silicon may result in a surface roughness having features that are less than about 60 to 20 nm Subsequent polishing steps may be desirable after a thin film is prepared by the ion implantation method. Finally, the ultimate thickness of the transferred thin film is dependent upon the ability of the ion implantation process to be precise. The depth resolution of implantation becomes progressively less accurate as the depth is increased. Accordingly, thin films prepared and transferred by an ion implantation method are impractical at thicknesses greater than 0.5 μm. Furthermore, the ions passing through the layer to be transferred create some damage in the perfect Si wafer which must be healed through annealing. This anneal step can limit the types of substrate to which the layer is transferred, in particular, the secondary substrate must be able to withstand the anneal temperature.
Various techniques for the preparation and transfer of crystalline thin film silicon for solar cells are known. For example, a porous silicon process (PSI) which starts with formation by electrochemical or other means of a porous silicon layer at the surface of a silicon wafer. Next the sample is annealed or otherwise caused to develop a surface which is suitable for subsequent epitaxy. A silicon film is then grown epitaxially on the porous substrate. After device fabrication a carrier glass may be attached to the front surface of the epitaxial layer and mechanical stress is used to split the resulting thin film from the substrate at the buried high porosity layer within the substrate. This technique relies upon the preparation of a substrate having low surface porosity and a high porosity at a selected depth which both permits high quality epitaxial growth and also facilitates subsequent detachment of the epitaxial layer. Thus, this method of preparing a thin crystalline silicon film for subsequent transfer has many steps and also utilizes some portion of the thickness of the substrate.
However, the layer transfer concept will only become economically viable for photovoltaic applications if the frequent reuse of the substrate is possible. Any method which inherently results in the loss of a substantial thickness of substrate material is thus economically problematic. The embodiments disclosed herein are directed toward overcoming one or more of the problems identified above.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.